yeah, it's hard with UGC because it relies on a "contain-intrinsic-size" attribute to tell the height of a div. That is impossible to say for posts, but maybe this could work for thread list pages. hmmmI'm not exactly sure how well it would work out for a forum software. Static websites it seems like it'd be easier enough to implement and make sense but a forum, not so sure.
I'm trying to think of how it'd work. Like hiding certain posts on the initial page load but what if users land directly on those posts due to last read, or most recent reply type of link (anchor links to specific posts).
It seems like it's literally just using CSS to target specific divs but not entirely sure! I'll look into a little further when I have time but this was my first reaction.